In sewage stations, septic tanks, wells, etc., it often occurs that solid matter or pollutants, such as socks, sanitary pads, paper, etc., clogs the submergible pump that is lowered into the basin of the system. The contaminations stick to the vanes of the impeller and become wound around the impeller.
In order to get rid of the clogging matter, it is known to equip centrifugal pumps with means for cutting up the solid matter. More precisely, the solid matter is cut up in smaller pieces between the vane of the impeller and a recess in the impeller seat of the pump housing, as is seen in for example DE 198,34,815 or U.S. Pat. No. 5,516,261. In each of the two referred documents it is just briefly shown how merely the edge between the leading edge of the vane and the tip of the surface of the vane of the impeller interacts with said recess. It is shown how said edge of the vane meets the cutting edge of the recess in a direction parallel to the direction of rotation of the impeller. More precisely, both cutting edges are perpendicular to the direction of rotation of the impeller. In these cases a superfluously high force, and thereby also a lot of energy, is needed to cut up the solid matter into smaller pieces.
If the solid matter is not cut up sufficiently efficiently into discrete pieces, but the pieces has long uncut fibers still connecting them to each other, the solid matter might clog the pump in an even more severe way. If the solid matter is semi-cut, as described, some pieces will get caught between the impeller and the pump housing and some pieces will still be to large to pass from the basin side of the impeller past the impeller. Thus, this will make the rotation of the impeller heavy and the energy consumption will increase. In a worst case scenario, the impeller will get totally jammed and thus the pump may get seriously damaged. Such an unintentional shutdown is costly, due to expensive and cumbersome and unplanned maintenance work.
DE 1,528,694 shows a pump comprising an impeller seat presenting a number of recesses of different shape and orientation, which in conjunction with the impeller improves the cutting action. Nevertheless, solid matter having long fibers is still a problem as the fibers may get tangled among the vanes of the impeller, resulting in a gradual decrease of the efficiency of the pump.
Another way of accomplishing the cutting up of the solid matter is shown in U.S. Pat. No. 3,096,718. Contrary to recesses, said document shows an impeller seat presenting a cutting blade, which has a sharp edge facing the vanes of the impeller and which in conjunction with said vanes cuts up the solid matter.
GB 1,125,376 and U.S. Pat. No. 5,516,261 shows a number of grooves extending in a spiral shape from a centrally located open channel in the impeller seat to the periphery thereof. The function of the grooves is, in conjunction with the vanes of the impeller, to transport the cut up pieces towards the outer wall of the pump housing and further out of the pump together with the pumped liquid. In order to ensure a proper function of the grooves, the solid matter has to be cut up into discrete pieces. Otherwise, if long fibers are uncut and connecting different pieces of solid matter, the pieces may be transported in different directions from the center of the impeller seat which may aggravate the clogging of the impeller.
From U.S. Pat. No. 3,128,051 it appears that instead of separate recesses for the cutting up of the solid matter and separate grooves for the transportation of the cut up pieces away from and past the impeller, it is possible to combine the two functions in a single element, which both presents the cutting edge of the recess and the transporting shape of the groove.
None of the abovementioned suggestions presents solutions to the drawbacks, or discuss the problems at all, related to the ability to cut long fibers.
EP 1,357,294 directed to the applicant, shows a pump which is exposed for solid matter included in unscreened sewage water, but which is not designed to cut up said solid matter. Instead the pump has a groove in the impeller seat for transportation of the entire contaminating subject towards the periphery of the pump housing. Further, the pump has a guide pin, the upper surface of which extends all the way from the surface of the impeller seat to the center of the impeller, and the function of which is to extend the function of the groove towards the center of an open channel in the impeller seat. Thus, there are no indications howsoever on how to ensure reliable cutting up of solid matter having long fibers.
Furthermore, submergible pumps are used to pump fluid from basins that are hard to get access to for maintenance and the pumps often operate for long periods of time, not infrequently up to 12 hours a day or more. Therefore it is highly desirable to provide a pump having long durability.